1. Field of the Invention
The present invention relates to the preparation of alkyl esters by the depolymerization of high molecular weight polyhydroxy acid (PHA) with an alcohol of 1-6 carbon atoms in the presence of an acid catalyst. More specifically, the present invention relates to the recovery of hydroxy acid value from a polyhydroxy acid polymer-containing source such as food container trash.
2. Description of the Related Art
Shaped articles of high molecular weight (at least 10,000, and normally 15,000 to 500,000 MW) polyhydroxy acids (PHA), particularly polylactic acid (PLA, polylactide), polyglycolic acid (PGA, polyglycolide), and copolymers thereof, have been known for years. An important property of these polymers is that they are slowly hydrolyzable and thereafter biodegradable to environmentally benign by-products. Consequently high molecular weight PHA polymer shaped articles are finding increasing application as replacements for polystyrene and other non-degradable polymers in products that will degrade in a landfill, such as fast food containers (Sinclair et al., WO90/01521, Feb. 22, 1990).
While this is a significant step in minimizing litter and long-term landfill disposal, discarding high molecular weight polyhydroxy acid articles for natural destruction by hydrolysis has the cost penalty of discarding the valuable polyhydroxy acid.
Although the hydrolysis of PHAs is well known, heretofore it has not been achievable in a time frame to permit recovery and reuse of the valuable hydroxy acid (HA) moities. In fact, although degradable, the time for degradation of high molecular weight PHAs is so long as not to offer a lessening burden on landfills.
Thus, there is a need for an economical method to recover and recycle the polyhydroxy acid content of this source of waste material and avoid burdening landfills with this waste.
The most economical routes for PHA production start with the acid such as tactic acid. The acid is converted to an ester, dimerized to a cyclic ring such as lactide, which is then polymerized to PHA. This is a complicated and costly process. See Bhatia U.S. Pat. No. 4,835,293 (May 30, 1989); Bellis U.S. Pat. No. 4,727,163 (Feb. 23, 1988); Klootwijk U.S. Pat. No. 3,578,700; Hostettler et. al. U.S. Pat. No. 3,284,417; and De Vries U.S. Pat. No. 4,797,468 (Jan. 10, 1989).
Bhatia U.S. Pat. No. 5,136,057 discloses the depolymerization of low molecular weight oligomers remaining after PHA polymerization. This patent application does not address the problem of recovery of the monomeric values from used high molecular weight PHA articles.
Copending and commonly assigned U.S. patent application Ser. Nos. 07/797,503, 07/796,273, 07/796,272 and 07/796,274 disclose the recovery of PHA's, respectively, in the presence of water and acid; in water under heat and pressure; in the presence of specific amines; and in the presence of water and lower alkyl alcohol.
The aforementioned patents and patent applications are incorporated herein by reference.